The serotonin neurotransmitter system and one of its key components, the serotonin transporter (SERT), is the primary target of this project. The development of the SERT knockout mouse in the LCS provided a new living tool to study SERT and serotonin receptors, the target molecules for the largest numbers of neuropsychiatric drugs used in the world. 30 plus neurochemical, behavioral and other phenotypic changes have been discovered in the serotonin knockout mouse. The resulting data is helping to guide the LCS and other laboratories in investigations of the multiple functional variants recently discovered in the human SERT gene. The broad goal of our studies is a better understanding of the serotonin neurotransmitter system and its contributions to physiology, behavior and human disorders, especially neuropsychiatric disorders. Serotonin has been implicated in almost every physiological function known. The serotonin transporter (SERT) recycles serotonin after its release, thereby terminating the action of serotonin at its receptors. In our attempt to better understand serotonin's function, we have generated a mouse model which either lacks the serotonin transporter or has a 50% reduction in serotonin transporter expression. These SERT -/- and +/- mice have gene-proportionate increases in extracellular fluid serotonin (5-HT) concentrations. i.e., 9- and 5-fold excesses respectively over +/+ mice, with the SERT deficiency present since conception. In models of anxiety, the SERT deficient mice exhibit markedly greater anxiety-like behaviors. Similarly, these mice have substantially greater activation of the HPA and sympathoadrenal systems in responses to minor stressful stimuli such as handling and saline injections, with high plasma ACTH and epinephrine increases, and more marked reductions in pituitary and adrenal gland hormones. The anxiety behaviors are restored to near normal levels by treatment with a serotonin 1A receptor antagonist. As serotonin has a trophic effect on neuronal cells, we are investigating the role of the serotonin transporter and certain neurotrophic factors such as BDNF in neuronal function, survival, and development. We have successfully developed a mouse SERT x BDNF double knockout mouse model to further investigate the role of the serotonin transporter and BDNF in the developing nervous system, as human genetic studies have separately implicated both genes in the development of affective and other neuropsychiatric disorders. Similarly, we have studied other gene x gene (epistatic) interactions of SERT with DAT, NET, and MAO genes in these mouse models.